1. Field of the Invention
The present invention relates to reducing code phase search space for a receiver in a distributed system.
2. Description of Related Art
A satellite positioning system comprises a set of orbiting satellites (also called space vehicles or ‘SVs’) that broadcast signals from which a receiver may determine its position. Two such systems are the NAVSTAR GPS system referenced herein (as described in Global Positioning System, Standard Positioning Service Signal Specification, second edition, Jun. 2, 1995, U.S. Coast Guard Navigation Center, Alexandria, Va.) and the Global Orbiting Navigational System (GLONASS) maintained by the Russian Republic. In order to determine its three-dimensional position within such a positioning system, a receiver must first acquire the signals of four SVs. The initial acquisition of each SV signal is typically computation-intensive and may take up to several minutes.
To acquire a GPS signal, a receiver must lock onto both the frequency of the carrier signal and the phase of the code modulated onto the carrier. Because of the motion of the SV relative to the receiver and the resulting Doppler shift, the frequency of the received carrier may change. Inaccuracies in the receiver's local oscillator may cause an additional frequency error. Therefore, locking onto the carrier may require the receiver to search for the signal across a range of frequencies.
Each SV transmits a signal that is spread by direct-sequence spread spectrum modulation. Specifically, each SV transmits a signal that is spread by a known digital pseudorandom (or ‘pseudonoise’) code called a coarse acquisition (CA) code. This periodic code has a chip rate of 1.023 MHz and repeats every 1,023 symbols (i.e. once per millisecond). A signal received at a receiver may be a composite of signals transmitted by several SVs.
The code phase of a received SV signal is established by the location in time of a predetermined position within the signal's CA code. As the CA codes are periodic, the possible locations of the predetermined position (i.e. the possible code phases) may be represented as points along the circumference of a circle, as shown in FIG. 1. Determining the code phase of a received signal requires searching for a correlation (e.g. between the receiver output and a code sequence based on the particular CA code) at each position on the circle until the code is located in the received signal (e.g. as indicated by the occurrence of a correlation peak).
Because the nominal carrier frequency of a GPS signal is 1.575 GHz, it may be difficult to maintain a signal lock in areas such as indoors, inside vehicles, and/or beneath tree canopies. When a portable GPS receiver loses a signal lock, an inconvenient suspension of positioning ability and a drain on computational resources may be sustained while the receiver attempts to reacquire the signal. As the frequency offset changes rather slowly, re-establishing a frequency lock after a short break may require only a limited effort. The code phase of a received signal changes more quickly, however, and it may be necessary to search for the lost signal across the entire 1,023-symbol code phase circle. For applications in which accurate position information must be available on demand, such a delay may be unacceptable. Of course, it may be beneficial to avoid long delays during initial acquisition as well.
It is desirable to augment certain wireless systems for mobile communications by adding the capability to locate the position of a particular mobile unit. One reason is a regulation promulgated by the Federal Communications Commission (Docket No. 94-102, third report and order adopted Sep. 15, 1999, released Oct. 6, 1999). This regulation requires all cellular carriers in the United States to be able to locate the position of a cellular telephone making a 911 call to within 50 meters for 67% of calls (and to within 150 meters for 95% of calls) by October 2001. Other uses for positional location capability in wireless communication systems include value-added consumer features such as navigation and vehicle fleet management support.
One option for adding position location to such a communications system is to add GPS receiving capability to the mobile units. However, such an approach suffers from the difficulty of maintaining reliable reception of GPS signals in many areas where mobile units are commonly used, such as indoors and within vehicles. On the other hand, the base stations in such a system are typically well-situated in terms of satellite visibility, and it may be feasible for the base stations to assist the mobile stations by collecting information on SV signals (including code phase) and forwarding it to the mobile stations.
In a code-division multiple-access CDMA system for wireless communications, operations by the mobile and base stations are synchronized to a common time base (see FIG. 1). Because of this feature, a base station can send code phase information relative to the time base that will be meaningful to a mobile station. Because of differences in the positions of the base and mobile stations, and because of inaccuracies in the mobile units' local oscillators, the code phase information sent by a base station may not coincide precisely with the code phase of the GPS signal received by a mobile unit. Nevertheless, such a procedure may substantially reduce the size of the code phase search base (for example, from 1,023 symbols to only 30).
In an analog system for wireless communications such as the advanced mobile phone service (AMPS) system widely used in the United States, however, no such time base exists between the mobile stations and the base station. Indeed, the operations of the stations may not be synchronized to within even one millisecond (i.e. the time to traverse an entire code phase circle). Therefore, no system reference point exists in relation to which the base stations may transmit meaningful code phase information (see FIG. 2). Therefore, in an AMPS system which supports GPS location ability by the mobile stations, any acquisition and reacquisition of a satellite lock may require searching the entire code phase circle. It is desirable to reduce the code phase search space in such distributed GPS systems.